As a type of proximity sensor for detecting presence (proximity) of an object of detection in a non-contact manner, there is known a high-frequency oscillation proximity sensor. FIG. 11 shows schematically an example of structure of a high-frequency oscillation proximity sensor of this type. As seen there, the high-frequency oscillation proximity sensor of this type has an oscillator circuit 2 including a detection coil 1. When an electrically conductive object S (for example, a metal object) is present near the detection coil 1, Q of the detection coil 1 changes. On the basis of this change, presence or proximity of the object S is detected.
Specifically, when the object S of detection is present near the detection coil 1, the resistance R and self-inductance L of the detection coil 1 changes due to electromagnetic induction occurring between the detection coil 1 and the object S. Due to this change, the oscillation amplitude and oscillation frequency of the oscillator circuit 2 changes. In the high-frequency oscillation proximity sensor, for example, the oscillation amplitude of the oscillator circuit 2 is detected by a detector circuit 3, and presence or proximity of the object S is detected on the basis of the detection output of the detector circuit 3 (the oscillation amplitude of the high-frequency oscillator circuit 2). Then, operation of an output circuit 4 is controlled, and, for example, a monitor-side load is selectively driven through a transistor 5 or an LED (light-emitting diode) 6 is driven to light to thereby make known (indicate) the presence or proximity of the object S. In FIG. 11, reference numeral 7 denotes a constant voltage circuit for supplying a drive voltage to the oscillator circuit 2, the detector circuit 3 and the like.
Regarding this type of high-frequency oscillation proximity sensor, it is required that not only detection characteristics should be stable but also a detection distance should be able to be long enough. Basically, these requirements can be met when temperature dependency of an internal resistance, i.e., a so-called copper resistance Rcu of the detection coil 1 is negated. Hence, for example, U.S. Pat. No. 4,509,023 and U.S. Pat. No. 4,942,372 propose a technique in which temperature compensation is made by applying a voltage proportional to the copper resistance Rcu, between the opposite ends of the detection coil 1.
FIG. 12 schematically shows a structure according to this technique. As shown there, a two-thread coil formed of two coil conductors each having a first end and a second end, joined together at their respective first ends and twisted together is used as a detection coil 1. One of the two coil conductors of the two-tread coil is connected to a resonance capacitor C1 and used as a resonance circuit coil L1, while the other coil conductor is used as an internal resistance compensation coil (copper resistance compensation coil) L2. Through an amplifier 8, a drive voltage Va is applied to the two-thread coil (detection coil) 1 so that the resonance circuit formed by the coil L1 and the capacitor C1 will oscillate. Also, the output of the amplifier 8 is fed back to the copper resistance compensation coil L2 with its phase turned by 90° through a capacitor C2.
In this circuit structure, however, it is necessary to find conditions for compensating for temperature dependency of the self-oscillation point of the resonance circuit, through regulating the resistance values of resistors R, P provided for the amplifier 8 and the capacitance of a capacitor C2 provided for voltage feedback. Also, the self-oscillation point needs to be optimized depending on the detection distance at which an object S should be detected. Thus, optimization of circuit constants is quite difficult. Particularly in order to negate the temperature dependency of the copper resistance Rcu of the detection coil 1, it is necessary to generate a voltage with an amplitude inversely proportional to the square ({overscore (ω)}2) of the angular frequency of {overscore (ω)} oscillation generated at the detection coil 1 and feed this voltage back to the copper resistance compensation coil L2. Thus, it is difficult to design a circuit which can surely negate the temperature dependency of the copper resistance Rcu, and therefore, it is difficult to stabilize the operating characteristics of the high-frequency oscillation proximity sensor.
A conventional common proximity sensor (proximity switch) is so arranged that when an object S of detection comes up to a certain distance, for example, the oscillator circuit 2 stops oscillating. The oscillator circuit 2 having an operating characteristic like this is generally called a hard oscillation circuit. In contrast, when an object S of detection should be detected at a plurality of points, or distances as it approaches, a so-called soft oscillation circuit having an operating characteristic such that the oscillation amplitude changes depending on the distance of the object S needs to be constructed.
The soft oscillation of the high-frequency oscillator circuit 2 means oscillation where the oscillation amplitude changes as the Q of the detection coil 1 changes. The Q of the detection coil 1 depends much on the internal resistance R of the detection coil 1 which changes depending on presence of an object S of detection. The Q of the detection coil is approximately given as [Q={overscore (ω)}L/R], where L is the self-inductance of the detection coil 1, and {overscore (ω)} is the resonance angular frequency of the LC resonance circuit formed by the detection coil 1 and the resonance capacitor C1. The change in Q of the detection coil due to presence of an object S can be represented by the ratio [Q ratio=Qin/Qout] of the value [Qin] which the Q takes when the object S is present near the detection coil 1 to the value [Qout] which the Q takes when the object S is not present.
In order to construct a soft oscillation circuit meeting the requirements that the operatiing characteristics should be stabilized and that the detection distance should be extended, it is necessary to change the oscillation amplitude of the high-frequency oscillator circuit depending on change in Q of the detection coil and also negate the temperature dependency of the copper resistance Rcu of the detection coil, for example, using a feedback circuit as mentioned above. This leads to problems such that the structure is complicated.